Merge pull request
#2231 from Carreau/ja
Improve Image format validation and add html width,height
This addresses the first two bullets of
#2053
Validates first argument to constrain format to png, jpeg,
Lets user specify width, height, or both and emits appropriate html
I introduced a couple of class constants to minimize the number of times I saw 'jpeg' and 'png'
and document the _ACCEPTABLE_FORMATS.
I added one real jpeg image to the test directory and one zero-length gif file to survive the file not found error in super()
Closes
#2053
Add `assert_is_none`, `assert_is_not_none` to `nose.tools`
